1. Field of the Invention
The present invention relates to systems and methods for navigation and control of spacecraft, and in particular to a system and method for determining the attitude of a spacecraft having at least one star sensor.
2. Description of the Related Art
Satellites enjoy widespread use for a variety of applications, including communications, surveillance, and data gathering. To perform their design mission, most satellites require accurate information regarding the spacecraft and payload attitude.
For applications where high bandwidth satellite attitude data is required, such data is typically obtained by on-board inertial measurement instruments such as inertial reference units having a plurality of gyros and accelerometers. However, while such instruments can provide high bandwidth information regarding the spacecraft attitude, they can only do so with respect to a datum attitude reference. Errors in this attitude reference propagate throughout the satellite navigation system. At the same time, it occasionally occurs that satellites lose their attitude reference, and must reacquire the reference with on-board sensors such as sun and earth sensors. When the satellite""s attitude datum has been entirely lost (a lost-in-space-scenario), the satellite must determine its attitude reference completely anew. When the accuracy of the attitude datum has been compromised, but not lost, (a confused-in-space scenario), the satellite must also determine its attitude reference anew.
Most attitude acquisition algorithms use a single star tracker to estimate the satellite attitude. Unfortunately, current attitude acquisition algorithms have no capability to deal with multiple star trackers. For example, the system disclosed in U.S. Pat. No. 5,745,869, issued to van Bezooijen on Apr. 28, 1998 and hereby incorporated by reference herein, is designed to work with data from a single star tracker.
The star acquisition method disclosed in the van Bezooijen patent uses an autonomous star tracker (AST). The method breaks up a star catalog into a xe2x80x9cGuide Star Databasexe2x80x9d (xcx9c4400 stars) and xe2x80x9cSupplementary (Auxiliary) Guide Star Databasexe2x80x9d (xcx9c8800 stars); the former including brighter stars and data concerning pairs of stars. The AST is used to track four bright stars to be found in the Guide Star Database. If four stars are unavailable, three stars from the Guide Star Database and one from the Supplementary Database are tracked. A pattern match is executed to identify stars. An attitude estimate is then formulated from the identified stars. Van Bezooijen""s method identifies stars by setting up xe2x80x9cmatch groupsxe2x80x9d (groups of stars with a xe2x80x9ckernelxe2x80x9d star in common), whose angular separation matches those of cataloged star pairs. Once the stars are identified, the attitude of the spacecraft can be determined. However, this technique requires the processing of multiple groups of multiple stars, increasing the number of required computations.
Further, many acquisition algorithms are designed to work with xe2x80x9csmartxe2x80x9d star trackers that are capable of using a built-in star catalog to determine which star is being tracked. Such an algorithm is disclosed in U.S. Pat. No. 5,412,574, issued to Bender, Parks, and Brozenec on May 2, 1995. Such systems cannot utilize minimum capability star trackers, such as the CT-602 available from the BALL Corporation, which can only identify star positions (in the star tracker reference frame) and corresponding star instrument magnitudes.
Hence, there is a need for a device and method for estimating the attitude of a spacecraft that can utilize data from multiple star trackers. There is also a need for a device and method that can determine spacecraft attitude from a single star pair. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method, apparatus, and article of manufacture for determining the attitude of a spacecraft having at least one star sensor, with such determination requiring only star sensor data. In the event that other attitude-related information is available, this invention can use such information to reduce processing time required to produce an answer. Furthermore, if more than one star sensor is available, the present invention can form an estimate of the sensor alignments.
The method comprises the steps of selecting a first reference star sensor from among the star sensors; designating two stars observed by the first reference star sensor as a primary star pair; identifying a candidate star pair corresponding to the primary star pair, wherein the candidate star pair is selected from a star catalog having a plurality of entries; estimating an inertial orientation of the first star sensor at least in part from the identified candidate star pair; determining the spacecraft attitude from the estimated inertial orientation of a reference star sensor selected from a group comprising the first star sensor. In one embodiment, the apparatus comprises means for performing the above-described method steps, and the article of manufacture comprises a program storage device tangibly embodying computer-readable instructions for performing the method steps.
Thus, the present invention has several advantages over the prior art. While the present invention may work with data from a single star tracker, it is designed to accept and use data from multiple star trackers, if observations from more than one tracker are available. If such is the case, the present invention can produce an estimate of the alignment of each tracker (i.e., the orientation of the sensor with respect to the spacecraft body, with respect to one or more xe2x80x9creferencexe2x80x9d tracker(s), in addition to the attitude estimate. Further, the present invention allows the use of lower capability star trackers that can only identify star position (in the tracker frame). If star instrument magnitude is available from the star tracker, the present invention will allow its use to produce attitude and alignment estimates faster than otherwise. If attitude-related information is available from some source, the present invention will allow its use to produce attitude and alignment estimates faster than otherwise.